There are various factors that needs consideration.

Firstly is how more expensive the OCed version compared to the normal version. If it is much more expensive, then there would be no point getting the OCed version as normally, an overclock GC would provide only a few additional fps which doesn't justify it's extra price.

Secondly is how high the OC is. If it is just a small OC, then that is also not worth it. The best is to compared the OCed version vs normal version and see the performance gains of the OC. Remember that the gains in fps is usually not great.

Thirdly, how high can a normal GC overclocked as compared to the OCed version. In some cases, the normal version can OC as high as the factory pre-overlocked version!! However, this would also depend on luck. If you're lucky, you can get one normal card that can OC higher. Like goldfries said, the OCed version would guarantee that it will work at that OC level.

Fourthly is how much you can afford. Some people would just buy the greatest of the series because they just have so much money to spend and want the highest frame rate obtainable!!!

I had a 7800 GS extreme edition which came pre-overclocked at 440/1300 (from stock 375/1200). It was much more expensive and could only be overclocked to 490/1450 stable. I had seen people with the normal stock version OC until 500/1500 or more!!!!!

So, in the end, IMO, there's no straight forward answer.....